It is well known in the prior art that an inner core and multi outer cores are separately manufactured and the outer cores are fixed to the inner core to form a rotor of an electric motor, wherein winding units (a bobbin and a winding wound thereon) are respectively formed on the outer cores. In such a prior art rotor, the outer cores are made of laminated metal sheets and the outer cores with the winding units are arranged at equal intervals in a circumferential direction.
There are, however, many kinds of manufacturing variations (e.g. weight variations) in the method of manufacturing the rotor of the above type. For example, the weight variations appear in the cores due to variations of press dies or a machine for laminating punch-out core pieces. The weight variations also appear in the winding units, because number of winding turns differ from each other among the adjacent winding units.
In the prior art rotor, in which the inner core and the outer cores are separately manufactured and then assembled together, there may be a problem of a lock of the rotor to be caused by the weight variations, when a center of gravity of the rotor is displaced during a rotation thereof.
In order to avoid such a problem, it is possible to adjust weight imbalance of the rotor by adding a plummet or cutting a portion of the rotor. However, it becomes higher in a manufacturing cost. Furthermore, in case of the fuel pump, pulsation may likely occur in a flow of fuel when the plummet is attached to an outer peripheral surface of the rotor or a portion of the outer periphery is cut away.